src/fuzzing.md - rust-lang/rustc-dev-guide - Git at Google

 # Fuzzing

 <!-- date-check: Mar 2023 -->

 For the purposes of this guide, *fuzzing* is any testing methodology that
 involves compiling a wide variety of programs in an attempt to uncover bugs in rustc.
 Fuzzing is often used to find internal compiler errors (ICEs).
 Fuzzing can be beneficial, because it can find bugs before users run into them.
 It also provides small, self-contained programs that make the bug easier to track down.
 However, some common mistakes can reduce the helpfulness of fuzzing and end up
 making contributors' lives harder.
 To maximize your positive impact on the Rust
 project, please read this guide before reporting fuzzer-generated bugs!

 ## Guidelines

 ### In a nutshell

 *Please do:*

 - Ensure the bug is still present on the latest nightly rustc
 - Include a reasonably minimal, standalone example along with any bug report
 - Include all of the information requested in the bug report template
 - Search for existing reports with the same message and query stack
 - Format the test case with `rustfmt`
 - Indicate that the bug was found by fuzzing

 *Please don't:*

 - Don't report lots of bugs that use internal features, including but not
   limited to `custom_mir`, `lang_items`, `no_core`, and `rustc_attrs`.
 - Don't seed your fuzzer with inputs that are known to crash rustc (details below).

 ### Discussion

 If you're not sure whether or not an ICE is a duplicate of one that's already
 been reported, please go ahead and report it and link to issues you think might be related.
 In general, ICEs on the same line but with different *query stacks* are usually distinct bugs.
 For example, [#109020] and [#109129] had similar error messages:

 ```
 error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:195:90: Failed to normalize <[closure@src/main.rs:36:25: 36:28] as std::ops::FnOnce<(Emplacable<()>,)>>::Output, maybe try to call `try_normalize_erasing_regions` instead
 ```

 ```
 error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:195:90: Failed to normalize <() as Project>::Assoc, maybe try to call `try_normalize_erasing_regions` instead
 ```

 However, they have different query stacks:
 ```
 query stack during panic:
 #0 [fn_abi_of_instance] computing call ABI of `<[closure@src/main.rs:36:25: 36:28] as core::ops::function::FnOnce<(Emplacable<()>,)>>::call_once - shim(vtable)`
 end of query stack
 ```
 ```
 query stack during panic:
 #0 [check_mod_attrs] checking attributes in top-level module
 #1 [analysis] running analysis passes on this crate
 end of query stack
 ```

 [#109020]: https://github.com/rust-lang/rust/issues/109020
 [#109129]: https://github.com/rust-lang/rust/issues/109129

 ## Building a corpus

 When building a corpus, be sure to avoid collecting tests that are already known to crash rustc.
 A fuzzer that is seeded with such tests is more likely to
 generate bugs with the same root cause.
 The simplest way to avoid this is to loop over each file in the corpus, see if it causes an
 ICE, and remove it if so.

 To build a corpus, you may want to use:

 - The rustc/rust-analyzer/clippy test suites (or even source code) --- though avoid
   tests that are already known to cause failures, which often begin with comments
   like `//@ failure-status: 101` or `//@ known-bug: #NNN`.
 - The already-fixed ICEs in the archived [Glacier] repository --- though
   avoid the unfixed ones in `ices/`!

 [glacier]: https://github.com/rust-lang/glacier

 ## Extra credit

 Here are a few things you can do to help the Rust project after filing an ICE:

 - [Bisect][bisect] the bug to figure out when it was introduced.
   If you find the regressing PR / commit, you can mark the issue with the label `S-has-bisection`.
   If not, consider applying `E-needs-bisection` instead.
 - Fix "distractions": problems with the test case that don't contribute to
   triggering the ICE, such as syntax errors or borrow-checking errors
 - Minimize the test case (see below).
   If successful, you can label the issue with `S-has-mcve`.
   Otherwise, you can apply `E-needs-mcve`.
 - Add the minimal test case to the rust-lang/rust repo as a [crash test].
   While you're at it, consider including other "untracked" crashes in your PR.
   Please don't forget to mark all relevant issues with `S-bug-has-test` once your PR is merged.

 See also [applying and removing labels][labeling].

 [bisect]: https://rust-lang.github.io/cargo-bisect-rustc/
 [crash test]: tests/compiletest.html#crash-tests
 [labeling]: https://forge.rust-lang.org/release/issue-triaging.html#applying-and-removing-labels

 ## Minimization

 It is helpful to carefully *minimize* the fuzzer-generated input.
 When minimizing, be careful to preserve the original error, and avoid introducing
 distracting problems such as syntax, type-checking, or borrow-checking errors.

 There are some tools that can help with minimization.
 If you're not sure how to avoid introducing syntax, type-, and borrow-checking errors while using
 these tools, post both the complete and minimized test cases.
 Generally,
 *syntax-aware* tools give the best results in the least amount of time.
 [`treereduce-rust`][treereduce] and [picireny][picireny] are syntax-aware.
 [`halfempty`][halfempty] is not, but is generally a high-quality tool.

 [halfempty]: https://github.com/googleprojectzero/halfempty
 [picireny]: https://github.com/renatahodovan/picireny
 [treereduce]: https://github.com/langston-barrett/treereduce

 ## Effective fuzzing

 When fuzzing rustc, you may want to avoid generating machine code, since this
 is mostly done by LLVM.
 Try `--emit=mir` instead.

 A variety of compiler flags can uncover different issues.
 `-Zmir-opt-level=4` will turn on MIR optimization passes that are not run by default, potentially
 uncovering interesting bugs.
 `-Zvalidate-mir` can help uncover such bugs.

 If you're fuzzing a compiler you built, you may want to build it with `-C
 target-cpu=native` or even PGO/BOLT to squeeze out a few more executions per second.
 Of course, it's best to try multiple build configurations and see
 what actually results in superior throughput.

 You may want to build rustc from source with debug assertions to find
 additional bugs, though this can slow down fuzzing by
 requiring extra work for every execution.
 To enable debug assertions, add this to `bootstrap.toml` when compiling rustc:

 ```toml
 rust.debug-assertions = true
 ```

 ICEs that require debug assertions to reproduce should be tagged
 [`requires-debug-assertions`].

 [`requires-debug-assertions`]: https://github.com/rust-lang/rust/labels/requires-debug-assertions

 ## Existing projects

 - [fuzz-rustc][fuzz-rustc] demonstrates how to fuzz rustc with libfuzzer
 - [icemaker][icemaker] runs rustc and other tools on a large number of source
   files with a variety of flags to catch ICEs
 - [tree-splicer][tree-splicer] generates new source files by combining existing
   ones while maintaining correct syntax

 [fuzz-rustc]: https://github.com/dwrensha/fuzz-rustc
 [icemaker]: https://github.com/matthiaskrgr/icemaker/
 [tree-splicer]: https://github.com/langston-barrett/tree-splicer/
	# Fuzzing

	<!-- date-check: Mar 2023 -->

	For the purposes of this guide, fuzzing is any testing methodology that
	involves compiling a wide variety of programs in an attempt to uncover bugs in rustc.
	Fuzzing is often used to find internal compiler errors (ICEs).
	Fuzzing can be beneficial, because it can find bugs before users run into them.
	It also provides small, self-contained programs that make the bug easier to track down.
	However, some common mistakes can reduce the helpfulness of fuzzing and end up
	making contributors' lives harder.
	To maximize your positive impact on the Rust
	project, please read this guide before reporting fuzzer-generated bugs!

	## Guidelines

	### In a nutshell

	Please do:

	- Ensure the bug is still present on the latest nightly rustc
	- Include a reasonably minimal, standalone example along with any bug report
	- Include all of the information requested in the bug report template
	- Search for existing reports with the same message and query stack
	- Format the test case with `rustfmt`
	- Indicate that the bug was found by fuzzing

	Please don't:

	- Don't report lots of bugs that use internal features, including but not
	limited to `custom_mir`, `lang_items`, `no_core`, and `rustc_attrs`.
	- Don't seed your fuzzer with inputs that are known to crash rustc (details below).

	### Discussion

	If you're not sure whether or not an ICE is a duplicate of one that's already
	been reported, please go ahead and report it and link to issues you think might be related.
	In general, ICEs on the same line but with different query stacks are usually distinct bugs.
	For example, [#109020] and [#109129] had similar error messages:

	```
	error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:195:90: Failed to normalize <[closure@src/main.rs:36:25: 36:28] as std::ops::FnOnce<(Emplacable<()>,)>>::Output, maybe try to call `try_normalize_erasing_regions` instead
	```

	```
	error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:195:90: Failed to normalize <() as Project>::Assoc, maybe try to call `try_normalize_erasing_regions` instead
	```

	However, they have different query stacks:
	```
	query stack during panic:
	#0 [fn_abi_of_instance] computing call ABI of `<[closure@src/main.rs:36:25: 36:28] as core::ops::function::FnOnce<(Emplacable<()>,)>>::call_once - shim(vtable)`
	end of query stack
	```
	```
	query stack during panic:
	#0 [check_mod_attrs] checking attributes in top-level module
	#1 [analysis] running analysis passes on this crate
	end of query stack
	```

	[#109020]: https://github.com/rust-lang/rust/issues/109020
	[#109129]: https://github.com/rust-lang/rust/issues/109129

	## Building a corpus

	When building a corpus, be sure to avoid collecting tests that are already known to crash rustc.
	A fuzzer that is seeded with such tests is more likely to
	generate bugs with the same root cause.
	The simplest way to avoid this is to loop over each file in the corpus, see if it causes an
	ICE, and remove it if so.

	To build a corpus, you may want to use:

	- The rustc/rust-analyzer/clippy test suites (or even source code) --- though avoid
	tests that are already known to cause failures, which often begin with comments
	like `//@ failure-status: 101` or `//@ known-bug: #NNN`.
	- The already-fixed ICEs in the archived [Glacier] repository --- though
	avoid the unfixed ones in `ices/`!

	[glacier]: https://github.com/rust-lang/glacier

	## Extra credit

	Here are a few things you can do to help the Rust project after filing an ICE:

	- [Bisect][bisect] the bug to figure out when it was introduced.
	If you find the regressing PR / commit, you can mark the issue with the label `S-has-bisection`.
	If not, consider applying `E-needs-bisection` instead.
	- Fix "distractions": problems with the test case that don't contribute to
	triggering the ICE, such as syntax errors or borrow-checking errors
	- Minimize the test case (see below).
	If successful, you can label the issue with `S-has-mcve`.
	Otherwise, you can apply `E-needs-mcve`.
	- Add the minimal test case to the rust-lang/rust repo as a [crash test].
	While you're at it, consider including other "untracked" crashes in your PR.
	Please don't forget to mark all relevant issues with `S-bug-has-test` once your PR is merged.

	See also [applying and removing labels][labeling].

	[bisect]: https://rust-lang.github.io/cargo-bisect-rustc/
	[crash test]: tests/compiletest.html#crash-tests
	[labeling]: https://forge.rust-lang.org/release/issue-triaging.html#applying-and-removing-labels

	## Minimization

	It is helpful to carefully minimize the fuzzer-generated input.
	When minimizing, be careful to preserve the original error, and avoid introducing
	distracting problems such as syntax, type-checking, or borrow-checking errors.

	There are some tools that can help with minimization.
	If you're not sure how to avoid introducing syntax, type-, and borrow-checking errors while using
	these tools, post both the complete and minimized test cases.
	Generally,
	syntax-aware tools give the best results in the least amount of time.
	[`treereduce-rust`][treereduce] and [picireny][picireny] are syntax-aware.
	[`halfempty`][halfempty] is not, but is generally a high-quality tool.

	[halfempty]: https://github.com/googleprojectzero/halfempty
	[picireny]: https://github.com/renatahodovan/picireny
	[treereduce]: https://github.com/langston-barrett/treereduce

	## Effective fuzzing

	When fuzzing rustc, you may want to avoid generating machine code, since this
	is mostly done by LLVM.
	Try `--emit=mir` instead.

	A variety of compiler flags can uncover different issues.
	`-Zmir-opt-level=4` will turn on MIR optimization passes that are not run by default, potentially
	uncovering interesting bugs.
	`-Zvalidate-mir` can help uncover such bugs.

	If you're fuzzing a compiler you built, you may want to build it with `-C
	target-cpu=native` or even PGO/BOLT to squeeze out a few more executions per second.
	Of course, it's best to try multiple build configurations and see
	what actually results in superior throughput.

	You may want to build rustc from source with debug assertions to find
	additional bugs, though this can slow down fuzzing by
	requiring extra work for every execution.
	To enable debug assertions, add this to `bootstrap.toml` when compiling rustc:

	```toml
	rust.debug-assertions = true
	```

	ICEs that require debug assertions to reproduce should be tagged
	[`requires-debug-assertions`].

	[`requires-debug-assertions`]: https://github.com/rust-lang/rust/labels/requires-debug-assertions

	## Existing projects

	- [fuzz-rustc][fuzz-rustc] demonstrates how to fuzz rustc with libfuzzer
	- [icemaker][icemaker] runs rustc and other tools on a large number of source
	files with a variety of flags to catch ICEs
	- [tree-splicer][tree-splicer] generates new source files by combining existing
	ones while maintaining correct syntax

	[fuzz-rustc]: https://github.com/dwrensha/fuzz-rustc
	[icemaker]: https://github.com/matthiaskrgr/icemaker/
	[tree-splicer]: https://github.com/langston-barrett/tree-splicer/